The present invention relates generally to integrated circuits, and, more particularly, to an oscillator circuit that generates a low temperature coefficient (LTC) clock signal.
Oscillator circuits such as a ring oscillator, a voltage-controlled oscillator (VCO), a relaxation oscillator, and the like, are used to generate periodic and oscillating signals. For example, a relaxation oscillator circuit is used in circuits such as DC/DC converters, counters, shifters, microcontrollers, and modulation circuits to generate a clock signal. Typically, a relaxation oscillator includes a reference current generator that generates a charge current, and an oscillator that generates an output clock signal based on the charge current. The oscillator includes a switch, set and reset capacitors, comparators, and some logic circuitry.
The set and reset capacitors are connected to the reference current generator via the switch to receive the charge current. The switch receives set and reset signals and provides the charge current to the set capacitor when the set signal is activate to charge the set capacitor to a first input voltage. The switch provides the charge current to the reset capacitor when the reset signal is activate to charge the reset capacitor to a second input voltage. One comparator is used to compare a reference voltage and the first input voltage from the set capacitor, and another comparator is used to compare the reference voltage with the second input voltage from the reset capacitor. The comparator outputs are XORed to generate an input clock signal, and the input clock signal is used to generate the set and reset signals, with one of the set and reset signals comprising the output clock signal.
However, various circuit components of the relaxation oscillator circuit such as the charge current, the reference voltage, capacitances of the set and reset capacitors, resistances of resistors, and the like, are sensitive to temperature variations, and the frequency of the output clock signal will vary with the circuit parameters. Thus, the output clock signal frequency varies significantly with variations in temperature, which is undesirable, because the relaxation oscillator circuit may provide an incorrect output clock signal that can cause the electronic circuit to malfunction.
One way to address temperature sensitivity is to include both positive temperature coefficient (PTC) and negative temperature coefficient (NTC) resistors in the relaxation oscillator circuit. The PTC resistors increase resistances to compensate for a decrease in circuit parameters of the corresponding NTC circuit components with an increase in temperature. The NTC resistors decrease resistances to compensate for an increase in the circuit parameters of the corresponding PTC circuit components with a decrease in temperature. The relaxation oscillator circuit fabricated using FD-SOI technology has optimized power consumption. However, it is difficult to fabricate NTC resistors using FD-SOI technology. Thus, relaxation oscillators that use both PTC and NTC resistors are difficult to fabricate using the FD-SOI technology.
It would be advantageous to have a relaxation oscillator that generates an output clock signal, is less sensitive to temperature variations, and has optimized power consumption.